DE4319569C1 - Method and appts. for prodn. of a semi-finished prod. - with smooth-rolling of the deposited metal ensures a small thickness tolerance - Google Patents

Abstract

A cleaned metal profile with low heat content is continuously fed, from bottom to top, through a bath (1) of molten metal of the same kind. Dependent on the height of the bath, the speed of the profile is adjusted so that its thickness is increased at least three-fold through depositions on its surface. After leaving the bath in an inert atmos., the strip passes through smoothing rolls (4) - whose vertical position is adjustable - when the average temp. T of the added metal satisfies the relation T=T1+a(T2-T1), where T1 is the solidus temp., T2 is the liquidus temp. and a is a factor varying from 0.1-0.8. The factor a lies within the range of values from 0.2-0.4. The thickness reduction of the strip by smooth-rolling varies from 5-15% while the final-to-initial thickness ratio of the strip lies in the range 3-7. The vertical position of the smoothing rolls (4) is adjustable by electro-mechanical or hydraulic means. The height adjustment installation includes at least one device for measuring the surface temp. of the strip. USE/ADVANTAGE - For mfr. of thin strips. The thickness tolerance of strips (with a width-to-thickness ratio in excess of 60) is securely maintained within 2%.

Description

The invention relates to a method for producing semi-finished products in the form thin metal strands according to the preamble of claim 1 and a device for performing the method.

EP 03 11 602 B1 discloses a method and a device for producing thin metal strands. A metal profile cleaned on the surface, for example in the form of a band-shaped steel sheet (mother band) with a thickness of 0.1-1.4 mm, is continuously guided through the bottom of a melt container filled with a similar type of steel melt. For this purpose, a slot-like opening is provided in the bottom of the melt container, which is provided with a sealing device in order to prevent melt from escaping. The temperature of the melt is close to the liquidus temperature T _liq . The steel strip is moved through the melt at a constant speed and led upwards out of the melt. Due to its low heat content (strip temperature approximately equal to room temperature), an adherent layer of crystallized and still liquid melt forms on the surface of the steel strip. The thickness of this layer can be a multiple of the thickness of the original mother tape. It depends in particular on the residence time in the melt (speed of the mother tape), on the melting temperature (temperature difference to the solidus temperature T _sol ), on the heat of fusion and the specific heat of the material used and on the thickness of the mother tape. The process must be carried out in such a way that re-melting of crystals that are already adhering is avoided. Under this condition there is a temperature gradient across the strip thickness. During the movement through the melt pool, the temperature inside the mother tape is the lowest and rises towards the edge. A temperature curve of the same quality is also present in the adhering layer. The liquidus temperature T _liq is precisely present in the outermost region of the layer.

The adhesive layer initially has a mixed composition formed crystals and in between existing molten Phase (mushy zone). The proportion of molten phases is decreasing outside towards. After leaving the melt pool, the adhesive cools Layer off, whereby the existing temperature gradient reverses. The adherent solidifies completely Layer.

It is also known from EP 03 11 602 B1, one in the above described semi-finished product to be produced after leaving the Melting pool in an atmosphere protecting against oxidation up to Cool or keep in until you enter a forming machine the semi-finished product is subjected to a hot and / or cold forming process becomes. A part of the production product quantity generated is then as Mother tape returned to the beginning of the procedure and again passed through the weld pool.  

The practical one is related to the production of steel strip material So far, using this procedure has been a major obstacle. Customers of high-quality cold or warm baths demand from the producer, among other things, compliance with a fluctuation range of Sheet thickness that is at most 2% of the nominal thickness. With the Such a close tolerance cannot be guaranteed with previous methods adhere. Existing irregularities in the thickness of the tape, the after leaving the melt pool and pass the prescribed The maximum can be exceeded by the following Practically no longer eliminate forming processes. This is because because of the extreme flatness of the in the rolling process used semi-finished product (width / ceiling ratio at least 60) Forming (with decreasing thickness) is practically only in the longitudinal direction and there is no significant spread anymore. Existing Differences in thickness along a line transverse to the longitudinal direction of the strip therefore remain relatively unchanged.

EP 0 311 602 B1 describes a second procedure, in the reverse direction from above into the weld pool introduced and withdrawn through the bottom of the melt vessel becomes. In this embodiment, the problem is floor sealing particularly serious since the directions of exit of the melt and Band material are the same and therefore not only a dynamic one Density effect is missing, but beyond that even a negative one Leakage tendency of the melt supports "entrainment effect" can be determined. For this reason it is special Sealing device in the form of a pair of sealing rollers in the floor area of the melt vessel is required. This pair of sealing rollers causes drastic squeezing of the "mushy zone" and with it a Squeeze out large parts of the liquid phase from the already formed "sponge-like" crystals. This has the consequence that  the thickness of the achievable adhesive layer compared to the first Process variant is significantly lower. Alone out Such a procedural approach comes from economic considerations hardly in question for a practical application.

The object of the invention is to provide a generic method in this regard to further develop that the required sheet thickness tolerance of maximum 2% can be safely observed, and a device for carrying out of the procedure.

With regard to the procedure, this task is solved by the characterizing features of claim 1. Advantageous Further developments of the invention are in subclaims 2 to 8 specified. An apparatus for performing the method, the basically also for the production of different types of profiles (e.g. round or any polygonal cross-sectional shape) is suitable, the Features of claim 9 and is characterized by the Features of subclaims 10 to 14 in an advantageous manner configurable.

In the following the invention based on the in the single figure schematically illustrated embodiment of one for the method suitable device explained in more detail.

A sheet coil 12 is used as the mother sheet, which is unwound at a certain speed. By the reference numeral 11 is a welding system band is indicated, which connects the end of an already unwound coil with a new coil 12, to allow a continuous process sequence. At 7 , a strip storage system is indicated, which can absorb a short-term standstill of the strip supply during the welding process when changing the coil, so that the production operation is not interrupted. In the production flow behind the belt storage system 7 , a belt cleaning 6 is arranged, in which the surface of the mother belt used is made metallically clean. A pair of transport rollers 2 ensures that the mother belt, which has a width / thickness ratio of at least 60, preferably at least 100, is guided into the melt 3 at a constant preselected speed through a corresponding slot-like opening in the bottom of the melt container 1 . The mother tape has a very low heat content, since it has room temperature, for example. The melt 3 (e.g. steel) consists of the same material as the mother tape. A seal, which is arranged on the bottom of the melt container 1 , is not shown separately in the figure. While the mother tape is passed through the melt 3 from bottom to top, a layer which grows with increasing dwell time (ie with an approach to the melt pool level) crystallizes, since the mother tape draws heat from the melt 3 in its immediate vicinity, whereby it heats up. The melt 3 is otherwise at a temperature of z. B. kept 10 K above the liquidus temperature. The level of the weld pool level is kept at a constant value by means of a feed, not shown. Taking these and other parameters into account (in particular solidus temperature, heat of fusion, specific heat of the melt material), the belt speed via the transport rollers 2 is preferably set such that the mother belt with the adhering layer has a thickness 3 to 7 times greater when leaving the melt 3 than the original mother band.

A smoothing roller device in the form of a pair of smoothing rollers 4 arranged next to one another is positioned above the melt pool level. The distance of this pair of smoothing rollers 4 from the melt pool level is variable in that the height of the pair of smoothing rollers 4 z. B. by an electromechanical or hydraulic adjustment device, which is indicated by the arrows, adjustable. The minimum distance of the pair of smoothing rollers 4 from the bath level is about 0.5 m, the maximum distance 5 m. The altitude is chosen so that the smoothing stitch takes place at a point where the layer adhering to the mother tape is on the one hand already relatively solidified, but on the other hand still has sufficient proportions of liquid phase in its outer zone, which also allows a problem-free flow of material transversely to the longitudinal direction of the mother band. It is therefore a question of the most favorable quantitative ratio of the solid to the liquid phase. The average temperature in the crystallized layer can be used as a control variable for this. According to the invention, the smoothing should take place at a temperature T _gl which satisfies the following relationship:

T _gl = T _sol + a × (T _liq - T _sol )

Therein, a means a factor in the range of 0.1-0.8, preferably in the range 0.2-0.4. The lower a is, the higher the solidified part. The lower limit is to be regarded as critical in that, in the case of malfunctions, complete or almost complete solidification can easily occur, which would make it impossible to compensate for any larger strip thickness differences. The upper limit of value a is primarily economic. Due to the high proportion of molten phase, a considerable part would be squeezed down because of the vertical guidance of the strip material, so that the output would be reduced accordingly. In order to facilitate the adjustment work, a strand surface temperature measuring device (not shown) can be provided in the adjustment range of the pair of smoothing rollers 4 . The smoothing roller pair 4 is advantageously provided with an internal fluid cooling (e.g. water cooling). The desired reduction in the thickness of the metal strand as a result of the smoothing stitch should be in a range of 5-15%.

In order to avoid an oxidation of the strand surface which is disruptive for the subsequent further processing of the semi-finished product produced, the adherent layer of the topsoil is protected against the entry of atmospheric oxygen by a housing 5 which can be flooded with an inert atmosphere. The housing 5 directly adjoins the melt container 1 and also envelops the pair of smoothing rollers 4 . An undesirable rapid cooling of the adhesive layer and thus to avoid substantial solidification, in case of need the Glättwalzeinrichtung 4 may be provided in particular in the field of adjustment, that at least parts of the walls of the enclosure are provided with a thermal insulation. 5 Otherwise, it is expedient to design the walls of the housing 5 as cooling walls, in particular as walls that are fluid-cooled from the inside (eg water cooling). Controlling the coolant temperature then allows controlled cooling of the semifinished product produced in the cooling zone 8 downstream of the smoothing roller device 4 , which leads to particularly favorable material properties. Similar to a continuous annealing process, the band-shaped material is guided in loops in a central section of the cooling zone 8 by corresponding deflection rollers, so that a correspondingly longer dwell time occurs in this zone. After the metal strand has undergone sufficient cooling, it leaves the housing 5 with its inert atmosphere and can, for. B. oiled by an electrostatic oiling device 8 and protected against corrosion. The material is then continuously wound into a coil 13 . After reaching a certain weight, the coil 13 is separated from the remaining strand by means of a pair of scissors 10 and transported away to a hot or cold rolling mill for further processing.

It is of course also possible, as already mentioned in the EP 03 11 603 B1 is described, further processing immediately to connect. In this case, the cooling can be used if necessary Saving thermal energy well above room temperature interrupted and the housing with an inert atmosphere until subsequent forming machine are performed.

The invention is illustrated by the following embodiment referred to the system diagram shown in the figure is explained in more detail.

A cold strip made of a steel X60

0.16% C
0.35% Si
1.30% Mn
0.013% P
0.003% S
0.041% Al
0.025% Nb
0.092% N
Remainder iron and usual impurities,

which had a thickness of 0.5 mm and a width of 100 mm, entered after degreasing in a pickling bath 6 with the aid of a pair of driving rollers 2 vertically through the bottom of a melt vessel 1 filled with liquid steel. The melt showed an analysis comparable to the steel strip. Liquid steel was fed continuously to the melt vessel 1 from a distributor (not shown). The height of the molten bath 3 and the speed of the steel strip are the control variables in order to set the desired contact time between the steel strip and the molten bath 3 , which should be about 2 seconds in the present case. Since the belt speed was 1 m / s, a melt pool height of 2 m was therefore maintained at all times. In the molten steel 3 , which had a temperature of approx. 1512 ° C, a crystallization of a total thickness of approximately 2.5 mm occurred during the passage of the steel strip, so that the total thickness of the steel strip as it emerged from the molten steel 3 was approximately 3 mm was. This steel strip with a "pasty" surface (two phases: melt and crystals) was then _shaped according to the formula T = T _sol + a × (T _liq -T _sol ) (here a = 0.5) with an average temperature of T = 1497 ° C + 0.5 × (1507 ° C-1497 ° C) = 1502 ° C in the grown layer in the vertically displaceable smoothing mill 4 , which in a z. B. Argon-filled and controlled cooled housing 5 was introduced, where its maximum thickness was reduced by approximately 17% (0.5 mm) and its surface roughness was largely reduced. For the present conditions, an integral temperature of 1502 ° C. for carrying out the smoothing stitch according to the invention proved to be particularly favorable in order to achieve the desired goal. The smoothing mill 4 was therefore adjusted in its vertical position so that this temperature was given on the entry level into the smoothing mill under the present cooling conditions. The smoothing stitch carried out resulted in a completely void-free steel strip with an optimally welded layering and a uniform thickness of approx. 2.5 mm. The existing deviation of the actual strip thickness from the target strip thickness was still only 1.6%, which is significantly below the maximum permissible value of 2% for hot strip, which is to be processed cold. After exiting the smoothing mill 4 , the steel strip, which was further protected against oxidation by an argon atmosphere, was subjected to controlled cooling in the water-cooled dome of the housing 5 and, after passing through a buffer space (cooling zone 8 ) which was also cooled and filled with argon, was passed to a winding station 13 fed. The steel strip was then rolled out to a thickness of again 0.5 mm in a cold rolling mill, not shown. The cold strip produced in this way had excellent mechanical-technological properties and met all the quality requirements. About 20% of the current production volume was returned to the process as input material.

It is surprisingly simple with the present invention possible to produce a ribbon-shaped metal strand that with respect its shape and surface tolerance extremely precise (deviation of the profile and the thickness over the length of the strip is less than 2%). At the same time, this process ensures a consistently safe Welding of the adhesive layer to the mother plate. Through the The possibility of controlled cooling can be a strip material achieve with excellent material properties.

Claims (14)

1. A process for producing semi-finished products in the form of thin metal strands, in particular made of steel, with thicknesses below 20 mm, in which an uncooled, cleaned metal profile of low heat content is continuously passed from bottom to top through a melt pool of the same type of material, the speed of the metal strand in Depending on the height of the molten pool is set so that depositing crystals and melt on the metal profile results in a strand thickness that corresponds to at least three times the original thickness of the metal profile and further maintaining an inert atmosphere during the production of the metal strand, characterized in that that for the production of semi-finished product having a width / thickness ratio of more than 60 and a fluctuation of the strip thickness of at most 2% the metal billet after leaving the melt bath a smoothing pass is subjected to when the average temperature in the initially crystallized Schich t of the metal strand T _gl fulfills the relationship: T _gl = T _sol + a × (T _liq - T _sol ) with a = 0.1-0.8
T _sol + solidus temperature
T _liq = liquidus temperature. 2. The method according to claim 1, characterized, that the factor a is in the range of 0.2-0.4.   3. The method according to any one of claims 1 to 2, characterized, that the thickness decrease in the smoothing stitch in the range of 5-15% lies. 4. The method according to any one of claims 1 to 3, characterized, that in the production of steel strands the speed at Passing the metal profile through the weld pool like this is set that the ratio of the strand thickness to original steel profile thickness is in the range of 3-7. 5. The method according to any one of claims 1 to 4, characterized, that the cooling of the metal strand through to the smoothing stitch through Influencing the wall temperature of an enclosure around the Melting bath and the lead out metal strand is controlled. 6. The method according to claim 5, characterized, that control in the sense of slowing down the natural Cooling down. 7. The method according to claim 5, characterized, that the control in the sense of an acceleration of the natural Cooling down.   8. The method according to any one of claims 1 to 7, characterized, that the metal strand after performing the smoothing stitch one controlled cooling is subjected. 9. The device, in particular for carrying out the method according to claim 1, with a melt container ( 1 ), in the bottom of which an opening provided with a sealing device preventing the escape of melt for the introduction of a metal profile is arranged, further with a transport device ( 2 ) for continuous passage of the metal profile through the device and with a housing ( 5 ) which covers the exit area of the metal profile from the melt ( 3 ) and an adjoining cooling zone ( 8 ) for the metal strand and can be filled with an inert atmosphere, characterized in that a smoothing roller device ( 4 ) is arranged within the housing ( 5 ) at a distance of 0.5-5 m from the bath level of the melt ( 3 ) and that the distance of the smoothing roller device ( 4 ) from the bath level is variable. 10. The device according to claim 9, characterized in that the opening is designed slot-like for the introduction of a strip-shaped sheet with a width / thickness ratio of at least 60 and that the smoothing roller device ( 4 ) is designed as a pair of smoothing rollers arranged side by side. 11. Device according to one of claims 9 to 10, characterized in that the height of the smoothing roller device ( 4 ) is adjustable electromechanically or hydraulically. 12. Device according to one of claims 9 to 11, characterized in that the housing ( 5 ) in the region of the height adjustment zone of the smoothing roller device ( 4 ) is at least partially designed with thermally insulating walls. 13. Device according to one of claims 9 to 12, characterized in that the walls of the housing ( 5 ) are designed at least in partial areas as fluid-cooled cooling walls. 14. Device according to one of claims 9 to 13, characterized in that at least one device for measuring the surface temperature of the metal strand is arranged in the region of the height adjustment zone of the smoothing roller device ( 4 ).